Conventionally heat-generation type airflow sensors are becoming the mainstream to measure the intake airflow, which are installed in an intake air passage of an internal combustion engine in automobile or the like, because such a type of sensor can detect mass airflow.
A sensor element can be formed as a thinner film partially by a semiconductor micromachining technique, whereby the airflow sensor can have high-speed responsivity. Hereinafter this thin-film part is called a diaphragm. On the diaphragm, a heating resistor and two or more thermosensitive resistors adjacent to the heating resistor are formed by patterning. The heating resistor is uniformly controlled to generate heat to be at a predetermined temperature or higher than the surrounding temperature, and the temperature distribution thereof is detected by the thermosensitive resistors. Since the temperature distribution changes with the amount of airflow passing over the sensor element, the variation in temperature distribution is detected by the thermosensitive resistors disposed upstream and downstream of the airflow direction, whereby the mass airflow can be measured.
As means for such a heat-generation type airflow meter using a sensor element, the sensor element and a lead frame to mount the sensor element thereon are surrounded with resin as a package by transfer molding, for example.
This is for reducing the number of components or the number of connections compared with the structure including a sensor element and a circuit mounted on a substrate made of ceramic or the like.
Such a sensor element and the thermal flow meter including such a packaged sensor element have the following problems.
To begin with, when stress is applied to the heating resistor and the thermosensitive resistors disposed on the diaphragm, their values of resistance change due to Piezoresistive effect, which becomes an erroneous cause of the mass airflow detected. If a pressure difference occurs between the surface and the rear face of the diaphragm part, the diaphragm part is deformed, so that stress is applied to the heating resistor and the thermosensitive resistors. To avoid this, there is a need to suppress such a pressure difference between the surface and the rear face of the diaphragm part.
As a method to reduce the pressure difference between the surface and the rear face of the diaphragm, Patent Literature 1 provides an opening at the surface of a diaphragm or at the rear face of a substrate to mount a sensor element thereon for communication between a cavity at the rear face of the diaphragm and the atmospheric pressure at the surface of the diaphragm.